As is known, the temperature during the various steps of processing, preserving and distribution of food products, drugs and the like (particularly those of the so-called “cold chain”) must be suitably controlled, since specific production specifications and rules of law establish specific temperature ranges for such steps, in order to ensure the quality and/or safety of the products.
For such purpose, it must be recalled that:                the “refrigeration” of a product, for example a food product, consists of its cooling and maintenance at a temperature typically from −1° C. to +8° C.; and        the “freezing” of a product corresponds with the cooling and maintenance of the product at temperatures lower than −18° C.        
Naturally, freezing slows, to a greater extent, the biochemical reactions that can cause the alteration of the products, with the consequence that the shelf-life of frozen products is longer, even though also in the presence of freezing temperatures several physical-mechanical changes can occur along with several biochemical reactions. In particular, this can occur in the cases where an even only temporary increase of the temperature takes place, e.g. to a level higher than −18° C.
It is therefore important to be able to control the temperature of a product to be preserved during the entire cold chain.
A solution known on the market for controlling the temperature of products, particularly refrigerated or frozen, is constituted by the so-called “thermochromic labels”. These are adhesive labels applicable to products to be monitored, including substances that irreversibly change color when their temperature—and thus that of the product to which they are applied—exceeds a certain threshold value.
Such labels, which must be preserved at temperatures lower than the respective threshold temperature, at which they change color, are disposable and are very costly.
Thermochromic labels are therefore not capable of giving information regarding the time interval of exposure of the respective product to temperatures higher than the threshold temperature.
The international application PCT/IB2010/055254 in the name of the applicant of the present patent application teaches a composition or suspension comprising a mixture of fatty alcohols, magnetite and polyvinylpyrrolidone (PVP) or polystyrene copolymer (PVP-PS copolymer) for producing a microencapsulated ink (obtained by mixing microcapsules containing the abovementioned magnetic suspension with a “base” for flexographic inks or serigraphic inks with UV crosslinking). The composition according to such international application has a number of drawbacks, and more particularly:                a “pasty” appearance;        a significant decrease over time of the sensitivity, caused by the partial incapacity of the PVP network to stabilize the nanoparticles of magnetite, which after a certain time period are progressively agglomerated together and can no longer be demagnetized due to the Brownian mechanism, which as is known only affects isolated SD (single domain) particles.        
The first of these problems negatively affects the microencapsulation: for the obtainment thereof, it is necessary to emulsify the hydrophobic fluid to be encapsulated. It will be understood that a pasty material is very difficult to disperse, unlike a fluid with low viscosity that tends to separate itself in the form of micro-droplets when stirred in the encapsulation liquid.
In addition, due to the affinity of water and the PVP, the encapsulation yield according to the method taught in the international application PCT/182010/055254 is low.
Due to the second problem or drawback, the composition that is the subject matter of the international application PCT/IB2010/055254 does not ensure constant performances over time. More particularly, if the composition is used for obtaining a sensor, if the latter is preserved for a long time above the threshold temperature (TTH) before being used, the composition tends to lose sensitivity over time.
The international application PCT/KR2004/003090 concerns nanoparticles obtained from a solution of a metallic salt of a surfactant in a suitable solvent, which yields, in that solvent, to a metallic-surfactant complex. The application is about a method for preparing nanoparticles, which, at the end of the method, are separated from a parent liquid. In PCT/KR2004/003090 no teaching of the use of PVB in the preparation of nanoparticles is disclosed.